The field of vision of a motor vehicle window, in particular a windshield, must be kept free of ice and condensation. In the case of motor vehicles with an internal combustion engine, a stream of air heated by engine heat can, for example, be directed to the windows.
Alternatively, the window can have an electrical heating function. From DE 103 52 464 A1, for example, a composite glass pane is known in which electrically heatable wires are placed between two glass panes. The specific heating output P, for example, roughly 600 W/m2, can be adjusted by the ohmic resistance of the wires. Because of design and safety aspects, the number of wires as well as the diameter of the wires must be kept as small as possible. The wires must not be visible or must be hardly perceptible in daylight and at night with headlight illumination.
Also known are transparent, electrically conductive coatings, in particular based on silver. WO 03/024155 A2 discloses, for example, an electrically conductive coating with two silver layers. Such coatings usually have sheet resistances in the range from 3 ohm/square to 5 ohm/square.
The specific heating output P of an electrically heatable coating with a sheet resistance Rsquare, an operating voltage U, and a distance h between two busbars can be calculated with the formula P=U2/(Rsquare*h2). The distance h between two busbars is, in typical windshields of passenger cars, roughly 0.8 m, which corresponds approx. to the height of the pane. In order to obtain a desired specific heating output P of 600 W/m2 with a sheet resistance of 4 ohm/square, an operating voltage U of roughly 40 V is necessary. Since the onboard voltage of motor vehicles is usually 14 V, a power supply or a voltage converter is necessary to generate an operating voltage of 40 V. A voltage increase from 14 V to 40 V is always associated with electrical line losses and additional costs for additional components.
US 2007/0082219 A1 and US 2007/0020465 A1 disclose transparent, electrically conductive coatings with at least three silver layers. In US 2007/0082219 A1, sheet resistances near 1 ohm/square are reported for coatings based on three silver layers. An operating voltage U=14 V, a sheet resistance Rsquare=1 ohm/square and a distance h=0.8 m yield a specific heating output P of roughly 300 W/m2.
To provide an adequate specific heating output P, for example, roughly 500 W/m2, in particular for heating relatively large panes, a further reduction of the sheet resistance of the electrically heatable coating is essential. This can be achieved with an electrically heatable coating with, typically, three silver layers by increasing the thickness of the individual silver layers. However, an excessive layer thickness of the silver layers results in inadequate optical properties of the pane, in particular with regard to transmittance and color appearance, such that legal regulations, as specified, for example, in ECE R 43 (“Uniform Provisions concerning the Approval of Safety Glazing and Composite Glass Materials”), cannot be complied with. Accordingly, the silver layers must be dimensioned such that the conductivity is high enough for adequate heating with simultaneously sufficient transmittance. The conductivity of the layers depends primarily on the crystallinity of the deposited silver.
Generally, the deposition of the layer system on a glass pane is done before the bending and the lamination of the windshield. Thus, the coating must have adequate thermal load capacity. However, the heating of the coated pane in the bending process results in oxidation of the silver layer. EP 2444381 A1 solves this problem through application of a blocker layer adjacent the silver-containing layer. This blocker layer serves, in particular, to stabilize the silver-containing layer during the thermal processing and improves the optical quality of the electrically heatable coating. EP 2444381 A1 discloses a blocker layer containing niobium, titanium, nickel, chromium, or alloys thereof, particularly preferably nickel-chromium alloys. However, a disadvantage of this solution is that the layer has an undesirable negative effect on the crystallinity of the silver-containing layer.